U.S. Pat. No. 4,817,560 which issued to Prince et al in 1989 discloses a pet litter box sifter. This reference discloses a grid-like structure which is intended for insertion into a pet litter box. The grid-like structure is left in place while the pet is using the litter box. When the pet is finished solid excrement is removed simply by raising the litter box sifter and then dumping the contents.
It is difficult to dump the excrement without having some spillage at the best of times, for the urine tends to bind the pet litter into clumps which become attached to the grid-like structure. The pet owner can get these clumps loose by banging on the side of the pet letter box sifter. However, if the pet litter box sifter is partially inverted when the clumps release the pet litter is scattered around, instead of in, the waste container into which the pet litter box sifter is being dumped.